This invention relates to an apparatus for picking-up an image of an object comprising a solid state image sensor having a number of image sensing elements and an optical system for forming an image of an object to be picked-up on the image sensor.
Such an image pick-up apparatus has been used in an endoscope, a television camera, etc. In the known image pick-up apparatus, the solid state image sensor has a flat shape and is arranged at a predetermined focal plane of the optical system. Such an image pick-up apparatus has a property that the apparatus can be constructed compact and small. For this purpose, the optical system, e.g. an objective lens is liable to have a wide angle of view. In general, when the angle of view becomes wider, aberrations, particularly curvature of field and distortion are increased. In order to correct such aberrations, it has been proposed to form the objective lens from a larger number of lens elements. This results in that the objective lens becomes large, heavy and expensive, which limits the merit of the image pick-up apparatus of this kind. Particularly, in case of installing the image pick-up apparatus in a distal end of an endoscope, it is impossible to arrange a large objective lens, so that the aberrations could not be corrected fully.
Further, in case of the endoscope, the object to be inspected has a large depth and object distances of various points on the object greatly differ from one another. Therefore, the best focal plane is curved to a great extent and thus, it is impossible to obtain the image of good quality up to its periphery.